Heater problems!

Jamessandlin

New Member
Aug 17, 2020
24
0
1
23
Kentucky
So recently just switched out my heater/ac controls, AC blows ice cold but with the heat it will blow hot for a about 3 seconds then go to room tempature and keep switching back and fourth, stays hot for 30 seconds and room tempature for 5 mins before getting hot again, is this most likely the thermostat?
 
  • Sponsors(?)


7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
5,380
1,729
194
Kearney, NE
This sounds more like a heater control valve or mix temp control door. Thermostats tend to fail and stay open, so you would usually not ever get good heat.
Are the controls still vacuum or all electronic in this generation of Mustang? I would think electric, but it sounds like a problem where it builds up enough vacuum to operate, then leaks it out, taking it’s time to build up enough vacuum to operate again.
When my similar year Grand Marquis had no heat, the flap, aka. mix door, was rubbing on the inside of the case and wore out the actuator.
 

Jamessandlin

New Member
Aug 17, 2020
24
0
1
23
Kentucky
The reason I was assuming it might be the thermostat is because the plastic piece / neck where the thermostat sticks has been leaking coolant so I know for sure either the gasket or thermostat or both already needs replaced just not sure if it’d cause the heat to act up
 

cbxer55

Active Member
Jan 25, 2020
307
65
38
60
Oklahoma
The reason I was assuming it might be the thermostat is because the plastic piece / neck where the thermostat sticks has been leaking coolant so I know for sure either the gasket or thermostat or both already needs replaced just not sure if it’d cause the heat to act up

That entire plastic thermostat housing is a POS. It's a known leak point due to a joint between the upper and lower parts. I replaced mine with a one piece metal unit. Took about an hour. It's also a good idea to replace the little short bypass hose on the underside while in there. But no, I doubt a leak at the thermostat will cause low heat inside. Only if the coolant level is so low as to be dangerous to drive.

 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
5,380
1,729
194
Kearney, NE
Thermostats are not designed to fail intermittently like this.
I second what @cbxer55 said. Unless the housing has leaked so much to be dangerously low, (and you have not topped the coolant off), you are probably not on the right track. The leak needs fixed now anyway. If you have air pockets in the system that bad, your engine is at serious risk of a big failure from low coolant, and the heater is not going to matter much at that point.
So replace the housing (or at least get it to reseal). Replace the thermostat while you are there. Do whatever your particular engine coolant system requires for air bleeding/burping. Then head back to diagnosing the heater issue.
 

Jamessandlin

New Member
Aug 17, 2020
24
0
1
23
Kentucky
I make sure to keep coolant in it, it doesn’t leak too much, engine never goes past halfway on the temp gauge, not sure if it’s the housing or the gasket around the thermostat itself, the heat or air hasn’t worked in awhile from the previous owner until I just replaced the heater controls